1. Technical Field
The present disclosure relates to coding elements, imaging apparatuses, and spectroscopic systems for acquiring spectral images and to spectroscopic methods in which such coding elements, imaging apparatuses, and spectroscopic systems are used.
2. Description of the Related Art
The use of spectral information of a number of narrow bands (e.g., several tens of bands or more) makes it possible to grasp detailed physical properties of an observation object, which has been impossible with conventional RGB images. Cameras for acquiring such multi-wavelength information are called hyperspectral cameras. Hyperspectral cameras are used in a variety of fields, including food inspection, biopsy, drug development, and mineral component analyses.
As an exemplary use of images acquired with wavelengths to be observed being limited to narrow bands, International Publication No. WO 2013/1002350 discloses an apparatus for distinguishing between a tumor site and a non-tumor site of a subject. This apparatus detects fluorescence at 635 nm from protoporphyrin IX accumulated in cancer cells and fluorescence at 675 nm from photo-protoporphyrin that are emitted in response to irradiation of pumping light. Thus, a tumor site and a non-tumor site are identified.
Japanese Unexamined Patent Application Publication No. 2007-108124 discloses a method for determining the freshness of perishables that decreases with time by acquiring information on the reflectance characteristics of continuous multi-wavelength light.
Hyperspectral cameras that can obtain multi-wavelength images or measure multi-wavelength reflectance can roughly be divided into the following four types:    (a) line-sensor-based hyperspectral cameras    (b) electrofilter-based hyperspectral cameras    (c) Fourier-transform-based hyperspectral cameras    (d) interference-filter-based hyperspectral cameras
(a) With a line-sensor-based hyperspectral camera, one-dimensional information of an object is acquired by using a member having a linear slit. Light that has passed through the slit is spot in accordance with the wavelengths by a dispersive element, such as a diffraction grating and a prism. The split light rays of the respective wavelengths are detected by an image sensor having a plurality of pixels arrayed two-dimensionally. This method allows only one-dimensional information of the object to be obtained at once. Thus, two-dimensional spectral information is obtained by scanning the entire camera or the entire measurement object in a direction perpendicular to the direction in which the slit extends. Line-sensor-based hyperspectral cameras have an advantage that high-resolution multi-wavelength images can be obtained. Japanese Unexamined Patent Application Publication No. 2011-89895 discloses an example of line-sensor-based hyperspectral cameras.
(b) An electrofilter-based hyperspectral camera that includes a liquid-crystal tunable filter (LCTF) and an electrofilter-based hyperspectral camera that includes an acousto-optic tunable filter (AOTF) are available. A liquid-crystal tunable filter is an element in which a linear polarizer, a birefringent filter, and a liquid-crystal cell are arranged in multiple stages. Light at unwanted wavelengths can be removed only by controlling the voltage, and light only at a specific desired wavelength can be extracted. An acousto-optic tunable filter is constituted by an acousto-optic crystal to which a piezoelectric element is bonded. Upon an electric signal being applied to the acousto-optic crystal, ultrasonic waves are generated, and compressional standing waves are produced inside the crystal. Through the diffraction effect of the standing waves, light only at a specific desired wavelength can be extracted. This method has an advantage that high-resolution moving image data can be obtained, although the wavelengths are limited.
(c) A Fourier-transform-based hyperspectral camera utilizes the principle of a two-beam interferometer. A light beam from an object to be measured is split by a beam splitter. The respective split light beams are then reflected by a stationary mirror and a movable mirror, recombined together, and detected by a detector. By temporally varying the position of the movable mirror, data indicating a change in the interference intensity that is dependent on the wavelength of light can be acquired. The obtained data is subjected to the Fourier transform, and the spectral information is thus obtained. The advantage of the Fourier-transform-based hyperspectral camera is that information on multiple wavelengths can be obtained simultaneously.
(d) An interference-filter-based hyperspectral camera utilizes the principle of a Fabry-Perot interferometer. A configuration in which an optical element having two surfaces with high reflectance that are spaced apart by a predetermined distance is disposed on a sensor is employed. The distance between the two surfaces of the optical element differs in different regions and is determined so as to match an interference condition of light at a desired wavelength. An interference-filter-based hyperspectral camera has an advantage that information on multiple wavelength can be acquired simultaneously in the form of a moving image.
Aside from the above-described methods, there is a method in which compressed sensing is used, as disclosed, for example, in U.S. Pat. No. 7,283,231. The apparatus disclosed in U.S. Pat. No. 7,283,231 splits light from an object to be measured by a first dispersive element, such as a prism, marks with a coding mask, and returns the path of the light ray by a second dispersive element. Thus, an image that has been coded and multiplexed with respect to the wavelength axis is acquired by a sensor. By applying the compressed sensing, a plurality of images of multiple wavelengths can be reconstructed from the multiplexed image.
The compressed sensing is a technique for reconstructing, from a small number of samples of acquired data, a greater number of pieces of data. When the two-dimensional coordinates of an object to be measured are (x,y) and the wavelength is λ, data f to be obtained is three-dimensional data of x, y, and λ. In the meantime, image data g obtained by the sensor is two-dimensional data that has been compressed and multiplexed in the λ-axis direction. The problem of obtaining the data f, which has a larger amount of data, from the obtained image g, which has a smaller amount of data, is a so-called ill-posed problem and cannot be solved as-is. However, typically, data of a natural image has redundancy, and by using the redundancy efficiently, this ill-posed problem can be transformed to a well-posed problem. JPEG compression is an example of techniques for reducing the amount of data by utilizing the redundancy of an image. JPEG compression employs a method in which image information is converted to frequency components and a nonessential portion of the data, such as a component with low visual recognizability, is removed. In the compressed sensing, such a technique is incorporated into an operation process, and the data space to be obtained is transformed into a space expressed by the redundancy. Thus, the unknowns are reduced, and the solution is obtained. In this transformation, for example, the discrete cosine transform (DCT), the wavelet transform, the Fourier transform, the total variation (TV), or the like is used.